3D printed electrospun nanofiber-based pyramid-shaped solar vapor generator with hierarchical porous structure for efficient desalination
Introduction
A growing number of people are facing the severe water shortage problem, and the trend is worsened along with the industrialization, climate change, population growth as well as water contamination. Making good use of abundant seawater on the earth to improve access to fresh water may alleviate the water scarcity crisis [1], [2], [3], [4]. Compared with the commonly used desalination methods such as reverse osmosis and multi-stage flash distillation with energy consumption and large equipment dependency, interfacial solar vapor generation merely utilizing clean and inexhaustible solar energy shows great promising attention [5], [6], [7], [8], [9]. Aiming at enhancing the water production rate of the technology, the solar vapor generator needs to satisfy the features of efficient light absorption, effective thermal energy utilization and plenty of water supply. Thus, various photothermal materials like plasmonic metals [10], [11], [12] and carbon-based nanomaterials [13], [14], [15] are widely used to integrate with different porous substrates to construct well-formed solar vapor generator.
Nanofibers with flexibility and modifiability [16], [17] have been considered as a suitable substrate for solar vapor generation from the perspective view of optimizing water transportation and thermal management [18], [19], [20], [21], [22], [23], [24]. As a basic form produced by electrospinning process, the as-prepared two-dimensional (2D) membranes which consist of nanofibers containing photothermal materials were directly used as the solar vapor generators [25], [26], [27]. In the recently reported research works, graphene oxide and carbon nanotubes (CNTs) have been co-electrospun with polyvinyl alcohol (PVA) and polyacrylonitrile (PAN) respectively to fabricate the composite photothermal nanofiber membranes [28], [29]. Besides, Janus nanofiber membrane can also be fabricated by sequential electrospinning for durable desalination by taking advantage of the unique hydrophobic/hydrophilic structure. For example, electrospun hydrophobic polymethylmethacrylate (PMMA) nanofiber membrane and hydrophilic PAN nanofiber membrane were used as the upper layer and lower layer respectively, which can ensure continuous evaporation without precipitation of salt on the Janus membrane [30].
However, the planar configuration can reach up the limit of evaporation rate and restrict the practical application [31]. Apart from the 2D membrane, aerogel or foam using the nanofiber as the building block has provided a promising choice to construct three-dimensional (3D) structural solar vapor generator [32]. When expanded the 2D membrane to 3D configuration, the abundant porous structure is instrumental in decreasing thermal conductivity and improving evaporation efficiency [33], [34], [35], [36], [37], [38], [39], [40]. For instance, Qiao et al. selected electrospun nanofiber membrane as the matrix and utilized gas-foaming method to fabricate the 3D closed-pore structural foam, which can float on the surface of water and achieve a higher evaporation rate [41]. Another focus point of the 3D nanofibrous material for solar vapor generation is salt-resistant function. Dong et al. developed the cellular structured aerogels based on silica nanofibers, and the resulting solar vapor generators embedded with CNTs or polypyrrole exhibited stable desalination performance in high-concentration brine [42], [43]. To sum up, although the nanofibrous materials have been widely used for solar vapor generation, the rational effective 3D structural design needs to enrich. Moreover, it still seems like a challenging task to achieve efficient evaporation yet not affect by salt deposition.
Herein, a 3D pyramid-shaped solar vapor generator (PSVG) was designed and fabricated by combining co-electrospinning PAN/CNTs nanofibers and the facile extrusion-based 3D printing technology (Scheme 1). From macroscopic and microscopic view, the concave-convex shape and hierarchical porous structure are beneficial to solar light absorption and vapor diffusion. Taking advantage of good photothermal conversion property and superior water transportation ability, the optimal solar vapor generator could achieve a quite high evaporation rate up to 2.62 kg m-2h−1 under one sun illumination. Furthermore, the convective flow could promote more vapor generation and escape relying on the crisscross channels. In the practical use for high-concentration brine treatment, the PSVG can still evaporate at a relatively higher rate and has good self-cleaning property. The 3D printed electrospun nanofiber-based PSVG presents a promising candidate for efficient desalination.
Section snippets
Chemical and materials
PAN powder (Mw = 86,000) was purchased from the Shanghai Chemical Fibers Institute. N,N-Dimethylformamide (DMF, AR) was provided by Sinopharm Chemical Reagent Co., ltd., China. Multi-walled CNTs (length of 10–30 μm) and sodium alginate (200 ± 20 mPa·s) were obtained from Aladdin Reagent Co., ltd. Sodium chloride (NaCl) and calcium chloride (CaCl2) were purchased from Shanghai Macklin Biochemical Co., ltd. All chemicals above were directly used without further treatment.
Preparation of the hybrid ink
The electrospun PAN/CNTs
Preparation of the nanofiber-based 3D printed sample
Fig. 1a systematically illustrates the overall preparation procedure of the nanofiber-based photothermal 3D printed sample. Firstly, PAN/CNTs nanofibers without beads were obtained rapidly by co-electrospinning using the ball-shaped nozzle, and CNTs were well encapsulated inside the nanofiber (Fig. S2a). The as-prepared membrane was then fully cut into short nanofiber segments of length less than 200 µm under the strong shear force (Fig. S2b). The resulting homogenized nanofiber dispersion was
Conclusion
In summary, a series of fine pyramid structural solar vapor generator was designed and fabricated by the facile extrusion 3D printing technology based on co-electrospinning PAN/CNTs nanofibers. The ideal characteristics of the materials suitable for solar vapor generation application including high light absorptance (98.3%), excellent hydrophilicity and evaporation rate (up to 2.62 kg m-2h−1 under one sun), have been attained by the optimum sample with the highest height, which beneficial from
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was partly supported by the grants (51973027, 52003044, 61902357 and 62171116) from the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities (2232020A-08), International Cooperation Fund of Science and Technology Commission of Shanghai Municipality (21130750100), and Major Scientific and Technological Innovation Projects of Shandong Province (2021CXGC011004). This work has also been supported by the Chang Jiang Scholars Program and
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